Magneto-optical recording/reproducing pickup head with a diffraction grating and a wollaston prism

ABSTRACT

A light source for generating light beams, a diffraction grating for splitting the light beam emitted from the light source into at least three light beams, and an objective lens for receiving the reflecting light beams from the recording medium, the magnification of the objective lens being −6.0 to less than −12.0, make up a collimatorless optical system. The reflecting light beams emanating from the objective lens are split by a beam splitter, these split light beams are each split into at least three light beams by a Wollaston prism, and are incident on a photo detecting element. A tracking error signal, a focusing error signal, and a magneto-optical signal are generated using the light beams received by the photo detecting element. The beam splitter and the Wollaston prism may be substituted by a multifunctional Wollaston prism. If the nine split light beams are all received by the photo detecting element, the signals produced by the photo detecting element are large in amplitude.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pickup device for recordinginformation to and reproducing the same from a magneto-optical recordingmedium, and also relates to a photo detecting unit in use with thepickup device.

2. Description of the Prior Art

A conventional magneto-optical recording/reproducing system detects amagneto-optical signal by using a magneto-optical light splittingelement by utilizing a birefringence, such as a 3-beam Wollaston prism,in an optical system with collimiator, viz., in parallel light beams.

FIG. 1 is a diagram showing an arrangement of a conventionalmagneto-optical recording/reproducing pickup device. A light beamemitted by a light source 1, such as a semiconductor laser device, isconverted, by a diffraction grating 2, into at least three spot lightbeams which will be used for generating a tracking control signal. Theselight beams are collimated by a collimating lens 3, usually consistingof two lenses joined together. The collimated light beams are convergedon a-magneto-optical disk 6 by an objective lens 5. To reproduceinformation, the plane of polarization is turned in accordance with aninverted magnetization pattern corresponding to information written intoa vertically magnetized film, in a recording track.

The light beams reflected by the magneto-optical disk 6 are renderedparallel by the objective lens 5 and returned to a beam splitter 4. Thelight beams are reflected by the beam splitter 4 toward a Wollastonprism 7. The Wollaston prism 7 separates the received light beams intoan S polarized light component, a P polarized light component, and alight component as the combination of the S and P polarized lightcomponents. These polarized light components are incident on a photodetecting unit 12 through a route of a reflecting mirror 8, a converginglens 9, a concave lens 10, and a cylindrical lens 11 for obtaining afocusing error signal. The direction of the magnetization of a readoutsignal surface is determined by comparing the intensities of the P and Spolarized light components. A focusing error signal is obtained, by theastigmatic method, from the light component as the combination of the Sand P polarized light components. A tracking error signal is obtained bycomparing the intensities of both sub-beams of the three beams derivedfrom the diffraction grating. Thus, the control signals are generatedfor controlling the focusing and tracking directions.

Another conventional optical pickup device in use with a magneto-opticalrecording/reproducing apparatus containing the optical system withcollimator is disclosed in Unexamined Japanese Patent Publication(Kokai) Sho-63-127436. In the publication, the parallel beams emanatingfrom the collimating lens are optical-axis transformed (reflected) by apolarizing beam splitter. The reflecting light beams are focused on themagneto-optical disk through an objective lens. The reflecting lightbeams from the magneto-optical disk are converted into parallel lightbeams by the objective lens. The light beams, after passing through thepolarizing beam splitter, are separated, by an analyzer, into an Spolarized light component, a P polarized light component, and a lightcomponent as the combination of the S and P polarized light components.Finally, a magneto-optical signal and other control signals forcontrolling the focusing and tracking directions are formed.

The conventional optical pickup devices including the optical systemwith collimator is employed in order to suppress a variation of thesplitting characteristics of the polarizing beam splitter 4 and theWollaston prism 7, and to reduce a degree of deterioration of themagneto-optical signals. The objective lenses used in parallel lightbeams can be more easily designed and manufactured than those used indivergent light beams. For this reason, the collimating lens 3, usuallyconsisting of two spherical glass-joined lenses, for collimating thedivergent light beams, and the converging lens 9 for converging theparallel light beams are indispensable for the conventional pickupdevices. Use of those lenses brings about complexity of theconstruction, increase of the number of the indispensable parts, andincrease of the size of the optical pickup device.

To solve the problems, there is proposed an optical pickup device in usewith the magneto-optical recording/reproducing apparatus, which isdesigned on the basis of an optical system without collimator, as shownin FIG. 2 (“O plus E”, No. 163, 1993, June, pp94 to 95). In the opticalpickup device, divergent light beams emitted from a light source 1 passthrough a convex lens 13 where a degree of the divergence of thedivergent light beams is reduced. The light beams as left divergent areincident, as an S polarized light, on a plate polarizing beam splitter14. The divergent light beams reflected by the plate polarizing beamsplitter 14 are converged on the recording surface of themagneto-optical disk 6, through an objective lens 15. The reflectinglight beams are converted, the objective lens 15, into the convergentlight beams which in turn enter the plate polarizing beam splitter 14.In the plate polarizing beam splitter 14, the polarizing film allowspart of the S polarized light beams and most of the P polarized lightbeams to pass therethrough. A half wave plate 17, located on the rearside of the plate polarizing beam splitter 14, turns the direction ofpolarization by 45° of the light beam. Thereafter, a plate analyzer 18splits the light beam into an S polarized light beam, a P polarizedlight beam, and a light beam as the combination of the S and P polarizedlight beams. These light beams are converted into electrical signals bya photo detecting unit 19.

In the pickup device shown in FIG. 2, to obtain exact information, it isnecessary to accurately adjust the angles of the plate polarizing beamsplitter 14 and the plate analyzer 18. This makes the assembling workdifficult. Further, a accurate control of the thickness of the plateanalyzer 18 is required. Accordingly, the manufacturing work isdifficult. The half wave plate 17 is provided for turning the plane ofpolarization by 45° and for disposing the plate analyzer 18 on a planewithout rotating along the optical axis. This half wave plate 17 isexpensive. Provision of the half wave plate 17 runs counter to the costreduction.

Additional pickup devices based on the optical system without collimatorare disclosed in Unexamined Japanese Patent Publication (Kokai)Hei-5-142419, Hei-5-142420, and Hei-5-142421. A Wollaston prism 21 asillustrated in FIGS. 3A and 3B is used. The optical system of theoptical pickup device is as shown in FIG. 3C. The Wollaston prism 21 asa multifunctional Wollaston prism includes a polarizing beam splitting21c. The polarizing beam splitting film 21c directs an incident lightbeam 24, which is emitted from a light source 1, toward the objectivelens 15, and allows a reflecting light beam 25, which comes in throughthe objective lens 15, to pass therethrough. (The polarizing beamsplitting film 21c is a multilayer film formed by alternately layering aplural number of dielectric thin films of different refractive indices,and is formed on the incident surface of the Wollaston prism 21.) TheWollaston prism 21 consists of a first prism 21a and a second prism 21b,both being made of crystalline and joined together along their longfaces. A plane including the optical axis of the reflecting light beam25 coming in through the objective lens 15 (the same thing iscorrespondingly applied to the optical axis of the incident light beam24 emitted from the light source 1) and the optic axis 21d of the firstprism 21a, is at an angle, not a right angle, to a plane including thatoptical axis and the optic axis 21e of the second prism 21b. TheWollaston prism 21 thus constructed is disposed slanted with respect tothe optical axis in the optical path of the reflecting light beams asnon-parallel light beams, whereby an astigmatism is caused.

The multifunctional Wollaston prism 21 splits the reflecting light beam25 into P polarized light beams b, S polarized light beams c, and thelight beams a as the combination of the S and P polarized light beams(FIGS. 3B). These light beams a, b, and c are received by photodetecting elements 16a, 16b, and 16c, respectively (FIG. 3C). A signalprocessor 16d compares the intensities of the light beams b and c,thereby reading information contained in the reflecting light beams. Thephoto detecting element 16a containing a 4-division photo diode iscapable of producing a focusing error signal as will be described later.

The multifunctional Wollaston prism 21, which is slanted to the opticalaxis, causes an astigmatism, and hence substitutes for the combinationof the polarizing beam splitter and the cylindrical lens. Use of thisprism contributes to reduction of the number of the required parts. Inthe Wollaston prism 21, which is constructed such that the optic axis21d of the first prism 21a is oriented at a right angle to the opticalaxis of the light beam passing therethrough, the image by the light beamemitted from the prism is not blurred.

In the construction using the multifunctional Wollaston prism 21 asshown in FIG. 3C, the pickup device produces an insufficient amount ofoutput power to a recording medium. The axially positioning adjustmentis essential to the photo detecting elements 16a, 16b, and 16c. Thisadjustment is laborious and difficult.

In writing information to and reading the same from a recording mediumby the optical pickup device as described above, the objective lens mustbe exactly positioned in both the focusing direction and the trackingdirection. In the magneto-optical disk as the recording medium, a Kerrrotation of the plane of polarization is read in the form of amagneto-optical signal. The magneto-optical signal is weaker than a pitsignal for the compact disk.

The size reduction of the magneto-optical disk system is a recent trendin this field of the products. The pickup device in use with themagneto-optical disk system is also under a constant pressure of sizereduction. In this circumstance, a unique optical pickup device has beenproposed (Unexamined Japanese Patent Publication (Kokai) Hei-4-157647).In the pickup device, the combination of a diffraction grating and a3-beam Wollaston prism, as already described, is used so as to allow asingle photo detecting element to receive the reflecting light beams andto pick up thereof information recorded in the magneto-optical disk (inthe form of magneto-optical signals), a focusing error signal indicativeof a positional deviation (defocusing quantity) in the focus direction,and a tracking error signal indicative of a positional deviation in thetrack direction.

In this pickup device, three split light beams, the light beam of the0-th order of diffraction, and the light beams of ±1st order ofdiffraction are incident on the recording layer of the magneto-opticaldisk. The reflecting light beams R0, R1, and R2 from the recording layerare applied to the 3-beam Wollaston prism 101 as shown in FIG. 4. The3-beam Wollaston prism 101 further splits each of these reflecting lightbeams into three reflecting light beams in a direction perpendicular tothe separation of the diffraction grating. Totally nine reflecting lightbeams R0, R1, R2, R10, R11, R12, R20, R21, and R22 are produced. Ofthose nine reflecting light beams, five light beams R0, R1, R2, R10, andR20 are detected by a single photo detecting unit 102 (FIG. 5). Theresult of the detecting is used for generating a focusing error signal,a tracking error signal, and a magneto-optical signal.

The conventional photo detecting unit 102 includes detecting elements103, 106 and 107 for detecting the light beams R0, R1, and R2corresponding to those of the 0th order and ±1st order of diffraction,which are split by the diffraction grating, and detecting faces 104 and105 for detecting the light beams R10 and R20, which are split by the3-beam Wollaston prism 101. The photo-detecting elements 103, 106 and107 for the light beams R0, R1, and R2 are disposed at a right angle tothe photo-detecting elements 104 and 105 for the light beams R10 andR20.

As seen, the reflecting light beams R11, R12, R21, and R22, located atfour corners, are not used in the conventional pickup device. The ratioof the quantities of the 0th order to ±1st order of diffraction, causedby the grating, is set at a relatively small value within a range from 4to 8, in order to increase the amplitude of the tracking error signal.The 3-beam Wollaston prism of which the basic split-light quantity ratioby the 3-beam Wollaston prism, i.e., ordinary ray:ray as the combinationof ordinary ray and extraordinary ray:extraordinary ray, is 25:50:25, isfrequently used. The quantity of each of the ordinary and extraordinaryrays is the half of that of the combination of the ordinary andextraordinary rays. The magneto-optical signal (ordinary rayintensity—extraordinary ray intensity) is relatively weak.

In the above-mentioned optical pickup device, the reflecting light beamsR11, R12, R21, and R22, located at four corners, are not used. Becauseof this, the tracking error signal is weaker than those by the lightbeams including those at the four corners. Since the ratio of thequantities of the light of the 0-th order of diffraction to the light ofthe ±1st order of diffraction, caused by the grating, is not large, theresultant focusing error signal and the magneto-optical signal are notlarge in amplitude. With regard to the basic split-light quantity ratioby the 3-beam Wollaston prism, the beam intensity ratio in the centralpart is large, while that on both sides is the half of that in thecentral part. The resultant magneto-optical signal is not large inamplitude.

A cubic beam splitter and a cylindrical lens are usually used in theconventional optical pickup device. The direction of the beam splittingby the grating is at a right angle to that of the beam splitting by the3-beam Wollaston prism. Accordingly, the photo detecting unit isconstructed such that the photo detecting elements for the trackingerror signal are arrayed at right angles to the photo detecting elementsfor the magneto-optical signal.

Use of the cubic beam splitter and a cylindrical lens inevitablyincreases the number of components and the size of the optical pickupdevice.

To cope with this, there is proposed an optical pickup device which usesa plate beam splitter with an astigmatism causing function (forgenerating a focusing error signal) for size reduction purposes (“O plusE”, No. 163, pp93 and 94). A diffraction grating is not used in thisoptical pickup system. The conventional photo detecting unit of the typein which the photo detecting elements for the tracking error signal isdisposed at a right angle to the photo detecting elements for themagneto-optical signal, is improperly operable when it is applied to theoptical pickup system using the diffraction grating and the plate beamsplitter. If applied, it fails to produce desired signals.

SUMMARY OF THE INVENTION

Accordingly, a first object of the present invention is to provide amagneto-optical recording/reproducing pickup device which includes anoptical system without collimator, and has various advantages, such as adecreased number of necessary parts, a simple construction, therequirements for the parts layout accuracy being not so high whencomparing with optical pickup device using the plate analyzer, a simpleassembling work, and use of a Wollaston prism, easily available, beingallowed.

A second object of the present invention is to provide a magneto-opticalrecording/reproducing pickup device which is small in size and low incost with a decreased number of required parts by unification of theWollaston prism, beam splitter, and an astigmatic generator element.Furthermore, the pickup device produces an increased output power to arecording medium, and allows an easy positioning of the photo detectingelement on the optical axis, and an easy designing of the objective lensby use of a positive singlet.

A third object of the present invention is to provide a photo detectingunit which produces tracking signal having large amplitude.

A fourth objective of the present invention is to provide an opticalpickup device using the photo detecting element of the third object.

A fifth object of the present invention is to provide a photo detectingunit which uses a plate beam splitter with an astigmatism causingfunction, and is well adaptable for an optical system in which thedirection of the beam splitting by the grating is at an angle, not aright angle, to that of the beam splitting by the 3-beam Wollastonprism.

A sixth objection of the present invention is to provide an opticalpickup device using the photo detecting unit of the fifth object.

According to first aspect of the invention, the magneto-opticalrecording/reproducing pickup device comprises: a light source forgenerating light beams; a diffraction grating for splitting the lightbeam emitted from the light source into at least three light beams; anobjective lens for converging the light beams emitted from the lightsource on a magneto-optical recording medium, and receiving thereflecting light beams from the recording medium, the magnification ofthe objective lens being −6.0 to −12.0 when an object point lies at thesignal surface of the magneto-optical recording medium; a beam splitterfor separating the light beams coming from the light source and incidenton the objective lens from the light beams coming through the objectivelens; a Wollaston prism for splitting the reflecting light beams comingfrom the magneto-optical recording medium through the beam splitter; anda photo detecting unit including a plural number of photo detectingelements for detecting the light beams emanating from the Wollastonprism. In the first magneto-optical recording/reproducing pickup device,a positive singlet for reducing a degree of divergence of the diverginglight beams from the light source may be provided between the lightsource and the objective lens.

In the first magneto-optical recording/reproducing pickup device, anoptical system without collimator in which a degree of convergence ofthe light beams incident on the beam splitter and the Wollaston prism isreduced can be constructed when the objective lens having themagnification within the above range of figures is used. The opticalsystem of the optical pickup device may be constructed as an opticalsystem without collimator by using the objective lens of thatmagnification, not collimating the light beams emitted from the lightsource. With this optical system, the optical pickup device succeeds insuppressing a variation of the splitting performances by the beamsplitter and the Wollaston prism, and reducing a degree of deteriorationof the magneto-optical signal. Further, the maximum value of thewavefront aberration measured on condition that the objective lens ismoved in the tracking and the focusing direction becomes smaller, thelarger a value of the magnification β is, as shown in FIG. 29. When themagnification is −6 or greater, it is easy to form an optical systemwhich satisfies the criterion value 0.07 λ of Marechal. A distance froma light source to a converging point on the disc, viz., the entirelength of the optical path ((L1+L2) in FIG. 6B), becomes longer, thelarger the magnification β is. If a distance L1 from the convergingpoint to the principal point is 3 mm or a bit longer, the total lengthof the optical path is approximately 40 mm when the magnification is−12. If the magnification exceeds this value, the total length of theoptical path becomes large.

For disposing the Wollaston prism, the incident plane thereof is merelyset perpendicular to the optical axis of the incident light beams.Moreover, the Wollaston prism may be disposed on a plane, and there isno need of using a polarization angle turning means, such as a half-waveplate. With provision-of the positive singlet between the light sourceand the objective lens, the distance between the light source and theobjective lens is reduced. This leads to the size and cost reduction ofthe optical pickup device, and an efficient use of the light beamemitted from the light source. For the aberration correction, thepositive singlet may be used in addition to the objective lens. Thisindicates an easy aberration correction of the optical system of thepickup device.

According to the second aspect of the invention, the magneto-opticalrecording/reproducing pickup device comprises: a light source forgenerating light beams; a diffraction grating for splitting the lightbeam emitted from the light source into at least three light beams; anobjective lens for converging the light beams emitted from the lightsource on a magneto-optical recording medium, and receiving thereflecting light beams from the recording medium, the magnification ofthe objective lens being −6.0 to −12.0 when an object point lies at thesignal surface of the magneto-optical recording medium; a Wollastonprism for causing an astigmatism including a polarizing light splittingfilm for separating the light beams coming from the light source andincident on the objective lens from the light beams coming through theobjective lens, the Wollaston prism being composed of first and secondcrystalline prisms which are joined, a plane including the optical axisof the reflecting light beam coming in through the objective lens andthe optic axis of the first prism, is at an angle, not a right angle, toa plane including that optical axis and the optic axis of the secondprism, the Wollaston prism is disposed in the optical path of thereflecting light beams as light beams being convergent and not parallelin a state that the incident plane thereof is slanted with respect tothe optical axis, whereby causing an astigmatism, a positive singleprovided between the light source and the objective lens, the totalmagnification of the optical system including the objective lens and thepositive singlet being −0.3 to −6.0; and a photo detecting unitincluding a plural number of photo detecting elements for detecting thelight beams emanating from the Wollaston prism.

In the second magneto-optical recording/reproducing pickup device, thediffraction grating and the positive singlet may be formed in a singleconstruction.

In the second magneto-optical recording/reproducing pickup device, acollimatorless optical system in which a degree of convergence of thelight beams incident on the Wollaston prism is reduced can beconstructed when the objective lens having the magnification within theabove range of figures is used. A degree of divergence of the divergentlight beams emitted from the light source is reduced by the positivesinglet, the optical path length is reduced, and an output power of theoptical pickup device to the object is increased. The optical system ofthe optical pickup device may be constructed as an optical systemwithout collimator by using the objective lens of that magnification.With this optical system, the optical pickup device succeeds insuppressing a variation of the splitting performances by the beamsplitter and the Wollaston prism, and reducing a degree of deteriorationof the magneto-optical signal. The Wollaston prism is disposed in theoptical path of the reflecting light beams in a state that the incidentplane thereof is slanted with respect to the optical axis, wherebycausing an astigmatism. For the aberration correction, the positivesinglet may be used in addition to the objective lens. This indicates aneasy aberration correction of the optical system of the pickup device.The light beams may be focused on the photo detecting unit by moving thepositive singlet along the optical axis. When the diffraction gratingand the positive singlet may be formed in a single construction, theturning of the diffraction grating or the positioning of the positivesinglet along the optical axis may be adjusted by a single operationmeans. Further, the maximum value of the wavefront aberration measuredon condition that the objective lens is moved in the tracking and thefocusing direction becomes smaller, the larger a value of the totalmagnification β′ is, as shown in FIG. 30. When the magnification isselected to be −3 or greater, it is easy to form an optical system whichsatisfies the criterion value 0.07 λ of Marechal. The main beam power tothe disk becomes smaller, the larger the total magnification β′ is. Thereason for this is that when the total magnification is large, thequantity of light beams entering the optical system from a light sourceis reduced. If the total magnification is −6 or smaller, the main beampower to the disk is 0.5 mW (the criterion value of MD) or larger.

To achieve the third object, there is provided a photo detecting unitfor receiving the reflecting light beams formed in a manner that a lightbeam is split into at least three light beams of the 0-th order and the±1st order of diffraction by a diffraction grating, the split lightbeams are incident on a recording medium, the light beams reflected onthe recording medium are each split into at least three light beams by aWollaston prism, the photo detecting unit comprising: a first photodetecting element for receiving the light beams of the +1st order ofdiffraction; and a second photo detecting element for receiving thelight beams of the −1st order of diffraction.

According to the fourth aspect of the invention, the magneto-opticalrecording/reproducing pickup device comprises: a light source forgenerating light beams; a diffraction grating for splitting the lightbeam emitted from the light source into at least three light beams ofthe 0-th order and the ±1t order of diffraction; an objective lens forconverging the light beams split by the diffraction grating on arecording medium, and receiving the reflecting light beams from therecording medium; a Wollaston prism for splitting the reflecting lightbeams coming in through the objective lens into at least three lightbeams, a photo detecting unit for receiving at least nine reflectinglight beams from the Wollaston prism, the photo detecting unit includinga first photo detecting element for receiving at least three light beamssplit by the Wollaston prism of the +1st order of diffraction, a secondphoto detecting element for receiving at least three light beams splitby the Wollaston prism of the −1st order of diffraction, a third photodetecting elements, consisting of a plural number of photo detectingelements, for receiving in divided form the central light beam of thethree light beams split by the Wollaston prism of the 0-th order ofdiffraction, and a pair of fourth photo detecting elements for receivingrespectively the reflecting light beams located on both sides of thecentral reflecting beam of the three light beams split by the Wollastonprism of the 0-th order; and lens drive means for driving the objectivelens for positioning adjustment.

In the fourth optical pickup device of the present invention, a trackingerror signal, for example, may be formed by calculating the differencebetween the output signals of the first and second photo detectingelements of the photo detecting unit. Since at least three reflectinglight beams together are incident on the first and second photodetecting elements, the tracking error signal generated is large inamplitude. A focusing error signal, for example, may be formed by theoutput signals of the divided photo detecting elements of the thirdphoto detecting element. A magneto-optical signal, for example, may beformed by calculating the difference between the paired fourth photodetecting elements.

Furthermore, when the ratio of the quantities of the 0th-order light tothe ±1st order of light is set at 8.5 or larger, the amplitude of thefocusing error signal and magneto-optical signal may be increased.However, if it exceeds 15, the amplitude of the tracking error signal istoo small. Therefore, a preferable ratio of the quantities of the0th-order light to the ±1st order of light is within the range of 8.5 to15. At the ratio within this range, the magneto-optical signal and thefocusing error signal may be increased in amplitude in a state that thetracking error signal is not too small in amplitude.

Moreover, the Wollaston prism for splitting the received light beam intoordinary ray, extraordinary ray, and the light beam as the combinationof the ordinary ray and extraordinary ray, is constructed such that theratio of the quantities of the ordinary ray and the extraordinary ray tothe whole light quantity is within 30 to 45%. When using such aWollaston prism, the produced magneto-optical signal is large. When thephoto detecting unit of the invention is further used, the trackingerror signal is also large. If the ratio of the quantities of theordinary ray and the extraordinary ray to the whole light quantityexceeds 45%, the focusing error signal is too weak.

A photo detecting unit, which achieves the fifth object of the presentinvention, receives at the photo detecting elements the light beams ofat least 3 by 3 in the form of a parallelogram, the received reflectinglight beams being formed in a manner that a light beam is split into atleast three light beams of the 0-th order and the ±1-st order ofdiffraction by a diffraction grating, the split light beams are eachsplit into at least three light beams by a Wollaston prism, the photodetecting element includes a first photo detecting element for receivingthe light beams of the ±1st order of diffraction, a second photodetecting element for receiving the light beams of the −1st order ofdiffraction, a third photo detecting element, consisting of a pluralnumber of photo detecting elements, for receiving in divided form thecentral light beam of the three light beams of the 0-th order ofdiffraction, and a pair of fourth photo detecting elements for receivingrespectively the reflecting light beams located on both sides of thecentral reflecting beam of the three light beams split by the Wollastonprism of the 0-th order, an angle between a line connecting the centersof the first and second photo detecting elements and a line between thepaired fourth photo detecting elements are not a right angle.

According to the sixth aspect of the invention, the magneto-opticalrecording/reproducing pickup device comprises: a light source forgenerating light beams; a diffraction grating for splitting the lightbeam emitted from the light source into at least three light beams ofthe ±1st order of diffraction and the light beam of the 0-th order ofdiffraction; an objective lens for converging the light beams split bythe diffraction grating on the recovery surface of a magneto-opticalrecording medium, and receiving the reflecting light beams from therecording medium, a Wollaston prism for splitting each of the reflectinglight beams coming in through the beam splitter into at least threelight beams, the direction of the splitting by the Wollaston prism beingat an angle to the direction of the splitting by the diffractiongrating; a photo detecting unit for receiving at least nine reflectinglight beams from the Wollaston prism, the photo detecting unit includinga first photo detecting element for receiving the light beams of the+1st order of diffraction, a second photo detecting element forreceiving the light beams of the −1st order of diffraction, a thirdphoto detecting element, consisting of a plural number of photodetecting elements, for receiving in divided form the central light beamof the three light beams of the 0-th order of diffraction, and a pair offourth photo detecting elements for receiving respectively thereflecting light beams located on both sides of the central reflectingbeam of the three light beams split by the Wollaston prism of the 0-thorder, an angle between a line connecting the centers of the first andsecond photo detecting elements and a line between the paired fourthphoto detecting elements being not a right angle; and lens drive meansfor driving the objective lens for positioning adjustment.

In the photo detecting unit, an angle between a line connecting thecenters of the first and second photo detecting elements and a linebetween the paired fourth photo detecting elements is not a right angle.Accordingly, the photo detecting element is well adaptable for anoptical system in which the direction of the splitting by thediffraction grating using the plate beam splitter capable of causing anastigmatism is at an angle, not a right angle, to the direction of thesplitting by the Wollaston prism.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an arrangement of a conventionalmagneto-optical recording/reproducing pickup device;

FIG. 2 is a diagram showing an arrangement of another conventionalmagneto-optical recording/reproducing pickup device;

FIG. 3A is a perspective view showing a 3-beams Wollaston prism used inan optical pickup device in use with a conventional magneto-opticalrecording/reproducing system;

FIG. 3B is a side view showing the Wollaston prism shown in FIG. 3A;

FIG. 3C is a diagram showing an arrangement of yet another conventionalmagneto-optical recording/reproducing pickup device incorporating theWollaston prism shown in FIGS. 3A and 3B;

FIG. 4 is a perspective view showing a 3-beams Wollaston prism;

FIG. 5 is a plan view showing a conventional photo detecting unit;

FIG. 6A is a diagram showing a magneto-optical pickup device accordingto a first embodiment of the present invention;

FIG. 6B is an explanatory diagram for explaining a magnification of anoptical system including an objective lens, a magneto-optical disk, anda photo detecting unit in the optical pickup device;

FIG. 7A is a perspective view showing a 3-beams Wollaston prism that maybe used for the pickup device of FIG. 6A;

FIG. 7B is a block diagram showing a readout signal determining circuitcoupled for reception with the Wollaston prism;

FIG. 8A is a diagram showing a distribution of light beams on a photodetecting unit in the optical pickup device;

FIG. 8B is a block diagram showing a tracking error detect circuit;

FIG. 8C is a block diagram showing a focusing error detect circuitcoupled for reception with a photo detecting element for detecting alight beam a in the photo detecting unit;

FIG. 9 is a diagram showing a magneto-optical pickup device according toa second embodiment of the present invention;

FIG. 10 is a diagram showing a magneto-optical pickup device accordingto a third embodiment of the present invention;

FIG. 11A is a diagram showing a magneto-optical pickup device accordingto a fourth embodiment of the present invention;

FIG. 11B is an explanatory diagram for explaining an optical systemincluding an objective lens and a positive singlet;

FIG. 12A is a perspective view showing a preferred structure of mountinga diffraction grating and a positive singlet in the optical pickupdevice of FIG. 11A;

FIG. 12B is a longitudinal sectional view showing the structure of FIG.12A;

FIG. 12C is a transverse sectional view showing the structure of FIG.12A;

FIG. 13 is a perspective view showing an optical pickup device accordingto a fifth embodiment of the present invention;

FIG. 14 is a diagram showing an optical system incorporated into theoptical pickup device shown in FIG. 13;

FIG. 15 is a diagram showing the diffraction by a diffraction gratingused in the optical system shown in FIG. 14;

FIG. 16 is a perspective view showing a distribution of light beams by a3-beam Wollaston prism and a diffraction grating used in the opticalsystem;

FIG. 17 is a plan view showing the construction of a photo detectingunit used in the optical system;

FIG. 18 is a block diagram showing a control system for the opticalpickup device of the fifth embodiment;

FIG. 19 is a diagram useful in explaining the principle of generating afocusing error signal by a cylindrical lens;

FIG. 20A is a diagram showing a spot formed on a third photo detectingelements-of the photo detecting unit;

FIG. 20B is a diagram showing another spot formed on a third photodetecting face of the photo detecting unit;

FIG. 20C is a diagram showing yet another spot formed on a third photodetecting face of the photo detecting unit;

FIG. 21 is a graph showing a variation of a signal value Fe for afocusing error signal with respect to defocusing;

FIG. 22 is a diagram showing the principle of reading data out of arecording layer of a magneto-optical disk;

FIG. 23 is a diagram showing an optical system of an optical pickupdevice incorporating a photo detecting unit according to a sixthembodiment of the present invention;

FIG. 24 is a perspective view showing a 3-beam Wollaston prism used inthe optical pickup device of FIG. 23;

FIG. 25 is a plan view showing a photo detecting unit which is a sixthembodiment of the present invention;

FIG. 26 is a diagram useful in explaining the principle of generating afocusing error signal by a parallel plate slanted with respect to theoptical axis;

FIG. 27A is a diagram showing a spot formed on a third photo detectingelement of the photo detecting unit;

FIG. 27B is a diagram showing another spot formed on a third photodetecting element of the photo detecting unit;

FIG. 27C is a diagram showing yet another spot formed on a third photodetecting element of the photo detecting unit;

FIG. 28 is a plan view showing a photo detecting unit which is amodification of the sixth embodiment of the present invention;

FIG. 29 is a graph showing variations of a wavefront aberration and amagnification β with respect to an entire length of the optical path;and

FIG. 30 is a graph showing variations of a wavefront aberration and atotal magnification with respect a main beam power to the disk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6A is a diagram showing a magneto-optical pickup device accordingto a first embodiment of the present invention. In the figure, referencenumeral 1 designates a light source as a semiconductor laser device; 20,a plane grating; 21, a plane polarizing beam splitter; 22, an objectivelens; 23, a 3-beams Wollaston prism; 26, a photo detecting unit; and 24,a concave lens movable in the optical axis as indicated by arrows z soas to focus the reflecting light beams on the photo detecting unit 26.

The plane grating 20 receives light beams from the light source 1 andgenerates a main beam and two subbeams located on both sides of the mainbeam, which are to be incident on the signal surface of themagneto-optical disk. These main and subbeams are used for forming atracking error signal. The plane polarizing beam splitter 21 receivesthe divergent light beams coming through the plane grating 20 from thelight source 1, and directs the divergent light beams toward theobjective lens 22. The objective lens 22 converges the reflecting lightbeams on the signal surface of the magneto-optical disk 6. On the signalsurface of the magneto-optical disk 6, the plane of polarization of theincident light beam is turned in accordance with a magnetization patternrepresentative of information recorded in a vertically magnetized film.The reflecting light beams with the turned plane of polarization fromthe magneto-optical disk 6 pass through the objective lens 22, and areincident, in the form of the convergent light beams, on the platepolarizing beam splitter 21. The plate polarizing beam splitter 21allows a part (e.g., 30%) of the S polarized light and most (e.g., 95%)of P polarized light to pass therethrough.

The 3-beam Wollaston prism as disclosed in Unexamined Japanese PatentPublication (Kokai) Sho-63-113503 is used for the Wollaston prism 23. Asshown in FIG. 7A, the Wollaston prism 23 is formed by joining togetherprisms 28 and 29 made of calcite or quartz, lithium niobate, etc. alongtheir long faces. A plane including the optical axis 27 of incidentlight and the optic axis 30 of the crystal prism 28 is at an angle otherthan a right angle to a plane including the optical axis 27 and theoptic axis 31 of the crystal prism 29. The Wollaston prism 23 thusconstructed receives the incident light beam and splits it into a lightbeam a as the combination of an ordinary light beam and an extraordinarylight beam, an ordinary light beam b and an extraordinary light beam c,both being slanted at angles to the incident light beam.

Under the condition that the rotation angle between the optic axis 30 ofthe crystal prism 28 and the optic axis 31 of crystal prism 29 is 45°,as shown in FIG. 7A, the intensities Ia, Ib, and Ic of the light beamsa, b, and c on the photo detecting unit are respectively 50%, 25% and25% of the intensity of the incident light beam when the plane ofpolarization of the reflecting light beam on the disk surface is notrotated. These light intensities Ia and Ib are: Ib>25% and Ic<25% orIb<25% and Ic>25% depending on the rotation direction of the plane ofpolarization of the reflecting light beam on the disk surface. As seen,information recorded in the magneto-optical disk can be discriminated byutilizing this nature, for example, by using a circuitry as shown inFIG. 7B. In the figure, the light intensities Ib and Ic are detected bydetectors 32 and 33 and then is compared by a comparator 34.

A distribution of light beams on the photo detecting unit 26 in theoptical pickup device is as shown in FIG. 8A. In the figure, light beamsa, b, and c result from the splitting of the main beam by the Wollastonprism 23, and light beams f and q, and hand i result from the splittingof the subbeams d and c by the Wollaston prism 23. The tracking errorsignal is formed by the known circuitry as shown in FIG. 8B. In thiscircuitry, the intensities of the subbeams e and e are detected bydetectors 35 and 36, and the detected signals are compared by acomparator 37. A tracking control signal is formed by using thistracking error signal. The control signal controls the movement of theobjective lens 22 in the radial direction of the disk 6 so that thelight beam is irradiated on a track.

In the photo detecting unit 26, a photo detecting portion thereof forreceiving the light beam a is divided into four parts A to D as shown inFIG. 8C. Photo detectors 40 to 43 are respectively located in thesedetecting elements A to D, which correspond to segmental parts A to D ofthe spot of the light beam a in FIG. 8A. The signals derived from thephoto detectors 40 to 43 are processed by operational circuits 44 to 46,whereby a focusing error signal (A+B)−(C+D) is produced. This method forfocusing error detection is known. The focusing error signal is used forcontrolling the axial movement of the objective lens 22 so that thelight beams are always focused on the recording surface of the disk.According to this embodiment, the plate polarizing beam splitter 21serves also as a cylindrical lens. When the focal point lies on thesignal surface of the disk, the spot of the light beam a is circular,and the focusing error signal (A+B)−(C+D) represents zero (0). Inanother state or a defocusing state, the focusing error signal takes anegative or positive value. The focusing error signal may also beobtained by a critical angle method, a knife edge method, a Foucaultmethod, or a beam size method.

To construct the optical pickup device as mentioned above, amagnification L2/L1 of the optical system including the objective lens22, the magneto-optical disk 6, and the photo detecting unit 26 isselected to be preferably within −6.0 to −12.0, more preferably within−7.0 to −9.0 (FIG. 6B). L1 is the distance between the object point andthe front principal point of the objective lens 22. The object point(the converging point of the light beam by the objective lens 22) is setat the signal surface of the disk 6. L2 is the distance between theimage point and the rear principal point of the objective lens 22. Inthe conventional optical pickup device for the optical disk, forexample, compact disk, an optical system without collimator having themagnification of approximately −4.0 to −6.0 is employed. However, inthis range of the magnification, some disadvantages arise. For example,the light splitting characteristics of the plate polarizing beamsplitter 21 varies although that of the plate non-polarizing beamsplitter utilizing in the conventional optical disk varies little. Themagneto-optical signal is deteriorated in quality in the Wollaston prism23. For this reason and an easy aberration correction, the preferablemagnification is −6.0 or larger (as absolute value) for this type of theoptical pickup device which cannot produce a large readout signal. Inview of miniaturization, −12.0 or larger of the magnification isunpractical since such a value elongates the length ranging from themagneto-optical disk 6 to the photo detecting unit 26.

Referring to FIG. 9, there is provided a diagram showing amagneto-optical pickup device according to a second embodiment of thepresent invention. The optical pickup device of the second embodiment ischaracterized in that a positive singlet 47 is additionally providedbetween the light source 1 and the plate polarizing beam splitter 21.According to this construction, the distance between the light source 1and the plate polarizing beam splitter 21 can be shortened. Because ofthis, the size reduction of the optical pickup device and an efficientuse of the light beam emitted from the light source 1 as well arerealized. For the aberration correction, the positive singlet 47 may beused in addition to the objective lens 22. This indicates an easyaberration correction in the overall optical system of the pickupdevice. There is no need of using a double glass-joined lens consistingof a pair of spherical lens for the positive singlet 47. It may be ameniscus lens of a spherical single lens, a plano-convex lens, or thelike.

FIG. 10 is a diagram showing a magneto-optical pickup device accordingto a third embodiment of the present invention. In the pickup device, adiffraction grating 20 and a prism polarizing beam splitter 25 allowdiverging light beams, which are emitted from the light source 1, topass therethrough to the objective lens 22. The light beams areconverted on the magneto-optical disk 6 by the objective lens 22, andreflected thereon. The reflecting light beams travels back through theobjective lens 22 and reach the prism polarizing beam splitter 25. Thelight beams are reflected by the prism polarizing beam splitter 25 anddirected toward the Wollaston prism 23. The light beams split by theWollaston prism 23 pass through the cylindrical lens 48 for forming afocusing error signal, and land on the photo detector unit 26. In thisoptical pickup device using the prism polarizing beam splitter, the comaof the light beams on the photo detecting unit is smaller than in thepickup device using the plate polarizing beam splitter.

FIG. 11 is a diagram showing a magneto-optical pickup device accordingto a fourth embodiment of the present invention. In the figure,reference numeral 1 designates a light source as a semiconductor laserdevice; 20, a diffraction grating for splitting a light beam receivedfrom the light source 1 into at least three light beams; 21, a Wollastonprism having a polarizing beam splitting film, which is slanted to theoptical axis as already described referring to FIG. 3C; 22, an objectivelens; 47, a positive singlet between the diffraction grating 20 and themultifunctional Wollaston prism 21; and 26, a photo detecting unit.

The plane grating 20 receives the light beams from the light source 1and generates a main beam and two subbeams located on both sides of themain beam, which are to be incident on the signal surface of themagneto-optical disk. These main and subbeams are used for forming atracking error signal. The positive singlet 47, which reduces a degreeof divergence of the light beams from the light source 1, allows thelight beams to travel to the Wollaston prism 21. The polarizing lightsplitting film 21c of the Wollaston prism 21 allows the S polarizedlight component and the P polarized light component of the linearlypolarized light incident thereon to pass therethrough at a preset ratioof them, or reflects those light components at a preset ratio. Forexample, it allows the P polarized light component of 100% and the Spolarized light component of 30% to pass therethrough, and reflects theS polarized light component of 70%. The divergent light beams reflectedthereby are directed toward the objective lens 22. The objective lens 22converges the reflecting light beams on the signal surface of themagneto-optical disk 6. On the signal surface of the magneto-opticaldisk 6, the plane of polarization of the incident light beam is turnedin accordance with a magnetization pattern representative of informationrecorded in a vertically magnetized film. The reflecting light beamswith the turned plane of polarization from the magneto-optical disk 6pass through the objective lens 22, and are obliquely incident on theWollaston prism 21 in the form of convergent light beams, and are splitby the Wollaston prism. The light beam is split into three light beamsa, b, and c by the 3-beams Wollaston prism 21, as shown in FIG. 3B. Theinformation is read out through the comparison of the light beams b andc, as described above. The intensities Ib and Ic of the light beams band c are changed depending on the direction of the turn of the plane ofpolarization on the disk surface as follows: Ib>Ic or Ib<Ic. The lightintensities Ib and Ic of the light beams are detected by the photodetecting elements 16b and 16c, and the output signals of thesedetecting elements are compared. The information is discriminated on thebasis of the comparison result.

Nine number of light beams a to i landing on the photo detecting unit 26are distributed as shown in FIG. 8A, as in the first embodiment of thepresent invention. The circuit of FIG. 8B compares the subbeams d and eto generate a tracking control signal. In the fourth embodiment, theWollaston prism 21, which is slanted to the optical axis, serves also asa cylindrical lens. Accordingly, a focusing error signal can be obtainedas in the first embodiment.

In the fourth embodiment, the positive singlet 47 is provided betweenthe light source 1 and the Wollaston prism 21. Accordingly, the distancebetween the light source 1 and the plate polarizing beam splitter 21 isreduced. This leads to the size reduction of the optical pickup device,an efficient use of the light beam emitted from the light source 1, andan increase of the output power to the disk 6. For the aberrationcorrection, the positive singlet 47 may be used in addition to theobjective lens 22. This indicates an easy aberration correction in theoverall optical system of the pickup device. The converging position ofthe light beams on the photo detecting elements of the photo detectingunit 26 may be adjusted by moving the positive singlet 47 along theoptical axis. This positioning is easier than in the case shown in FIG.3C where the photo detecting elements 16a to 16d are positioned.

In the fourth embodiment, the magnification L2/L1 of the objective lensis preferably −6.0 to −12.0, more preferably −7.0 to −9.0.

A total magnification L3/L1 of the optical system including the positivesinglet 47 is preferably −0.3 to −6.0, more preferably −3.5 to −5.0(FIG. 11B). If the magnification L3/L1 is smaller than −3.0, theaberration correction is difficult. If it exceeds −6.0, an insufficientquantity of the light is collected from the light source 1, so that anecessary amount of output power to the disk cannot be secured.

FIGS. 12A to 12C cooperatively show a preferred structure of mountingthe diffraction grating 20 and the positive singlet 47 in the opticalpickup device. As shown in FIGS. 12A and 12B, the diffraction grating 20and the positive singlet 47 are mounted in a tubular case 50 of whichthe inside 52 is empty. An operation pin 51 is radially attached to thetubular case 50. In this structure, the operational pin 51 is operatedin the direction R for the adjustment of the direction (indicated by anarrow R) of the turn of the diffraction grating 20. It is operated inthe direction X for the adjustment of the movement of the positivesinglet 47 along the optical axis (indicated by X). More specifically,the operational pin 51 is inserted into a hole 50a radially elongated inthe tubular case 50. The operational pin 51 is passed through a hole 54aof a housing 54. As shown in FIGS. 12B and 12C, a plate spring 53secured to the housing 54 presses the tubular case against the corner ofthe housing 54 in a state that the tubular case is rotatable about itsaxis and movable in the optical axial direction. In FIG. 12A, referencenumeral 55 designates an incident light.

Use of the unit structure of the diffraction grating 20 and the positivesinglet 47 brings about various advantageous effects, such as easyassembling, easy adjustment, simple construction.

A fifth embodiment of the present invention will be described.

FIG. 13 is a perspective view showing an optical pickup device accordingto a fifth embodiment of the present invention. FIG. 14 is a diagramshowing an optical system incorporated into the optical pickup deviceshown in FIG. 13. The optical system is the same as the optical systemusing the Wollaston prism of the conventional optical pickup deviceshown in FIG. 1.

An optical pickup device 110 of the fifth embodiment, as shown in FIG.13, includes a lens drive unit (lens drive means) 112 for positioning anobjective lens 111 to be described later in the focusing direction Z andin the track direction X, and an optical system block 113 containingoptical parts the like.

The construction of the optical system block 113 is shown in FIG. 14. Inthe construction, a semiconductor laser device 114 as a light sourcegenerates laser beams B. A diffraction grating 115 receives the laserbeams B from the light source 1 and splits them into at least threeillumination light beams B₀, B₁, B₂. A collimating lens 116 collimatesthe three light beams B₀, B₁, B₂ emanating from the diffraction grating115. A beam splitter 117 changes the direction of the optical axis ofthe reflecting light beams R (R₀, R₁, R₂) emanating from the objectivelens 111. The objective lens 111 receives the illuminating light beamsB₀, B₁, B₂ coming through the beam splitter 117 from the light source 1and converges the light beams on the recording layer (recording portion)on the magneto-optical disk D as a recording medium, and also receivesthe reflecting light beams R₀, R₁, R₂ from the magneto-optical disk D. A3-beams Wollaston prism 118 receives the reflecting light beams R₀, R₁,R₂, which are received by the objective lens 111 and the beam splitter117 and changed in their optical axis by the beam splitter 117, andsplits each of those three light beams into three light beams, totallynine light beams R′ (R₀, R₁, R₂, R₁₀, R₁₁, R₁₂, R₂₀, R₂₁, R₂₂). Areflecting mirror 119 reflects the reflecting light beams R′. Aconverging lens 120 converges the reflecting light beams R′ reflected bythe reflecting mirror 119. A concave lens 121 reduces a converging angleof the reflecting light beams R′ incident thereon. A cylindrical lens122 causes an astigmatism for the reflecting light beams R′. A photodetecting unit 123 detects the reflecting light beams R′ transmittedthrough the cylindrical lens 122.

The diffraction grating 115, as shown in FIG. 15, diffracts the incidentlight beams B into at least three illuminating light beams; a 0-th order(of diffraction) light beam B₀, a +1st order light beam B₁, and a −1storder light beam B₂. Of those illuminating light beams B₀, B₁, B₂, thelight beams B₁ and B₂, located on both sides of the light beam B₀, areused for generating a so-called tracking error signal.

The 3-beams Wollaston prism 118, as shown in FIG. 16, is a cubic prismformed by joining a couple of triangle prism 118a and 118b along theirslanted surfaces 118c. Each triangle prism, shaped like a right-angledtriangle in cross section, is made of a uniaxial crystal, such asquartz, rutile, lithium niobate or calcite. The crystal axes of thetriangle prisms 118a and 118b are at right angles to the optical axis ofthe reflecting light beams R, and at about 45° to each other. When thereflecting light beams R₀, R₁, R₂ pass through the slanted surfaces 118cof the triangle prisms 118a and 118b of the 3-beam Wollaston prism 118,these beams are refracted in different directions depending on thedirections of the polarization thereof. As a result, these beams aresplit into first group of light beams R₁₀, R₁₁, R₁₂ as ordinary ray, asecond group of light beams R₂₀, R₂₁, R₂₂ as extraordinary ray, and athird group of light beams R₀, R₁, R₂ as the combination of the ordinaryray and the extraordinary ray.

The photo detecting unit 123 is constructed as shown in FIG. 17. In thephoto detecting unit 123, a first photo detecting element 125 receivesthree reflecting light beams R₁, R₁₁, R₂₁, which are the +1st order (ofdiffraction) reflecting light beams split by the 3-beams Wollaston prism118. A second photo detecting-element 126 receives three reflectinglight beams R₂, R₁₂, R₂₂, which are the −1st order (of diffraction)reflecting light beams split by the 3-beam Wollaston prism 118. A thirdphoto detecting element 124 consists of 4-divided photo detectingelements 124a to 124d which receive in divided form the centralreflecting light beam R₀ of three reflecting light beams R₀, R₁₀, R₂₀,which are the 0-th order reflecting light beam split by the 3-beamWollaston prism 118. Paired fourth photo detecting elements 127 and 128receive respectively the reflecting light beams R₁₀ and R₂₀, located onboth sides of the central reflecting beam R₀. Thus, the reflecting lightbeams R₁₁, R₁₂, R₂₁, R₂₂, which were conventionally not used, can bedetected by the photo detecting unit of the invention, so that thetracking error signal having large intensity is output. These photodetecting elements 124a to 124d, and 125 to 128 produce photo detectingsignals representative of intensities of the detected reflecting lightbeams and transfers them to a signal processing unit 130 to be describedlater, by way of lead wires, not shown.

FIG. 18 is a block diagram showing a control system for the opticalpickup device of the fifth embodiment.

The optical pickup device 110 of the present embodiment, as shown,includes a control unit 129 (as control means) for controlling theoptical pickup device 110 per se. The control unit 129 is coupled withthe lens drive unit 112. The photo detecting elements 134 to 128 areconnected through the signal processing unit 130 to the control unit129.

The signal processing unit 130 generates a tracking error signal and afocusing error signal using the photo detecting signals from the photodetecting elements 124 to 126, and transfers them to the control unit129. The same generates a magneto-optical signal using the photodetecting signals from the photo detecting elements 127 and 128, andtransfers it to an output buffer 131.

The tracking error signal is generated by calculating the differencebetween the signal from the first photo detecting element 125 whichreceives the reflecting light beams R₁, R₁₁, R₂₁, and the second photodetecting element 126 which receives the reflecting light beams R₂, R₁₂,R₂₂.

An astigmatism method is used for generating a focusing error signal. Avalue Fe of the focusing error signal is given by the following equation(1)

Fe=(Ra+Rc)−(Rb+Rd)   (1)

where Ra, Rb, Rc, and Rd are the photo detecting signals from the4-divided photo detecting elements 124a to 124d.

As shown in FIG. 19, when the disk D lies at a focal point (indicated bya continuous line), the reflecting light beams from the magneto-opticaldisk D is focused at a point Z₀ when viewed in the Y axis directionperpendicular to the paper surface. The same is focused at a point X₀when viewed in the Y direction parallel to the paper surface. Therefore,the cross section of the beam is circular at a point P. If the 4-dividedphoto detecting elements 124a to 124d is located at the point P, acircle spot (R₀) is formed on the third photo detecting element 124 asshown in FIG. 20A. In an in-focus state, the value Fe of the focusingerror signal is 0.

When the objective lens 111 approaches to the disk D (indicated bydotted lines), the focal point moves to a point Z′ and X′. The beamcross section at the point P becomes fat in the Y direction, while itbecomes thin in the x direction. The spot (R₀) on the third photodetecting circuit 124 is an ellipse oblique to the right upper corner inthe coordinates shown in FIG. 20B, and the value Fe of the focusingerror signal is positive.

When the objective lens 111 is apart from the disk D (indicated bydotted lines), the spot (R₀) on the third photo detecting element 124 isan ellipse oblique to the right lower as shown in FIG. 20C, and thevalue Fe of the focusing error signal is negative.

Accordingly, the result of calculating the equation (1) shows a state ofthe objective lens 111, in-focus, too close to the disk, or too partfrom the disk. As seen from FIG. 21, one can know a degree of defocusingof the objective lens 111 from the signal value Fe.

The control unit 129 receives a tracking error signal and a focusingerror signal from the signal processing unit 130, and controls the lensdrive unit 112 in accordance with these signals, and hence moves theobjective lens 111 in the focusing direction and the tracking direction,thereby properly positioning the objective lens.

The lens drive unit 112 includes a focusing coil and a tracking coil.Currents controlled by the control unit 129 for the adjustment ofproperly positioning the objective lens 111, are fed to these coils,whereby the objective lens 111 is moved in the focusing direction X andthe tracking direction Z as shown in FIG. 13, and properly positioned.

Of the reflecting light beams R₀, R₁₀, R₂₀, which correspond to the 0-thorder reflecting light beams, the reflecting light beams R₁₀ and R₂₀ arereceived by the paired fourth photo detecting elements 127 and 128 andthe difference between them is calculated, in order to generate amagneto-optical signal in the signal processing unit 130.

The operation of the present or fifth embodiment will be described withreference to the related drawings already referred to and FIG. 22.

The laser beams B emitted from the light source 114 are split into firstto third illuminating light beams B₀, B₁, B₂ by the diffraction grating115, and converted into parallel light beams by the collimating lens116, pass through the beam splitter 117, and are incident on theobjective lens 111.

The objective lens 111 converges the illuminating light beams B₀, B₁, B₂on the recording layer of the magneto-optical disk D.

The recording layer of the magneto-optical disk D is verticallymagnetizable. Each magnetic domain of the recording layer is magnetizedin its direction. The light beams B₀, B₁, B₂, when reflected on therecording layer, are turned in their planes of polarization inaccordance with the directions of the magnetization (this phenomenon isknown as the so-called Kerr effect). This phenomenon is illustrated inFIG. 22.

The reflecting light beam R from a magnetic domain in the recordinglayer where is magnetized in an upward direction (the domain magnetizedin the direction indicated by an upward arrow), viz., the magneticdomain storing data “0”, has the plane of polarization, which is turnedin the plane of polarization by +θ_(k) from that of the light beams B₀,B₁, B₂ incident on the recording layer. The reflecting light beam R froma magnetic domain where is magnetized in the direction as the inverteddirection (the domain magnetized in the direction indicated by adownward arrow), viz., the magnetic domain storing data “1”, has theplane of polarization, which is turned in the plane of polarization by−θ_(k) from those of the light beams B₀, B₁, B₂ incident on therecording layer.

Thus, the light beams B₀, B₁, B₂ are incident on the recording layer ofthe magneto optical disk D, and reflected thereon. The reflecting lightbeams R₀, R₁, R₂, which are turned in the planes of polarizationdepending on the content of the data recorded, are incident again on theobjective lens 111.

The reflecting light beams R₀, R₁, R₂ are converted into parallel lightbeams by the objective lens 111, and enter the beam splitter 117. Thereflecting surface 117a of the beam splitter 117 directs toward the3-beams Wollaston prism 118.

The 3-beams Wollaston prism 118 splits each of those three light beamsinto three light beams, totally nine light beams R′ (R₀, R₁, R₂, R₁₀,R₁₁, R₁₂, R₂₀, R₂₁, R₂₂), and these split beams are incident on thereflecting mirror 119.

The reflecting light beams R′ are reflected by the reflecting mirror119, then converged by the converging lens 120, undergo an astigmatismin the cylindrical lens 122, and land on the photo detecting unit 123.

The photo detecting elements 124a to 124d, and 125 to 128 of the photodetecting unit 123 produce photo detecting signals representative ofintensities of the detected reflecting light beams and transfers them tothe signal processing unit 130.

The signal processing unit 130 generates a tracking error signal, afocusing error signal, and a magneto-optical signal using the photodetecting signals from the photo detecting unit 123, and transfers thetracking error signal and the focusing error signal to the control unit129, and the magneto-optical signal to an output buffer 131.

The control unit 129 receives a tracking error signal and a focusingerror signal from the signal processing unit 130, and controls the lensdrive unit 112 in accordance with these signals, an hence moves theobjective lens 111 in the focusing direction Z and the trackingdirection X, thereby properly positioning the objective lens 111. Themagneto-optical signal is used as a reproduced data signal.

A photo detecting unit according to a sixth embodiment of the presentinvention will be described with reference to FIGS. 23 to 28. An opticalsystem shown in FIG. 23 is substantially the same as that of the firstembodiment. The operations of a diffraction grating 215, a 3-beamWollaston prism 218, and the like are substantially the same as those ofthe corresponding ones in the fifth embodiment shown in FIG. 14. Thelight beams B₀, B₁, B₂ directed from the diffraction grating 215 to thepolarizing light beam splitter 216 are slanted at about 45° with respectto the direction perpendicular to the paper surface in a planeperpendicular to the optical axis.

In the sixth embodiment, the polarizing light beam splitter 216functions as both a reflecting plate and an astigmatism causing element.The conventional optical pickup device uses a cylindrical lens as anastigmatism causing element and a cubic beam splitter for splitting thereflecting light beam coming from the magneto-optical disk D into aplural number of light beams which in turn strike the photo detectingunit. In this embodiment, one component substitutes for these twocomponents. This contributes to size reduction of the optical pickupdevice. The deformation direction of the light beams due toastigmatization by the polarizing light beam splitter 216 are orientedin the horizontal X direction and vertical direction Y direction on thephoto detecting elements on the photo detecting unit 223 as shown inFIG. 23.

The 3-beam Wollaston prism 218, as shown in FIG. 24, is a cubic prismformed by joining a couple of triangle prism 218a and 218b along theirslanted surfaces 218c. Each triangle prism, shaped like a right-angledtriangle in cross section, is made of a uniaxial crystal, such asquartz, rutile, lithium niobate or calcite. The crystal axes of thetriangle prisms 218a and 218b are at right angles to the optical axis ofthe reflecting light beams R, and at about 45°, for example, to eachother. The reflecting light beams R₀, R₁, R₂ entering the 3-beamsWollaston prism 218 are those illuminating light beams B₀, B₁, B₂ whichare split, while being slanted at about 45°, by the diffraction grating215 and reflected on the magneto-optical disk D. Accordingly, thoselight beams are incident on the 3-beam Wollaston prism 218 in a statthat those split light beams are slanted, as shown. When passing throughthe slanted surfaces 218c of the triangle prisms 218a and 218b of the3-beam Wollaston prism 218, these beams are refracted in differentdirections depending on the directions of the polarization thereof. As aresult, these beams are split into first group of light beams R₁₀, R₁₁,R₁₂ as a P polarized light component, a second group of light beams R₂₀,R₂₁, R₂₂ as an S polarized light component, and a third group of lightbeams R₀, R₁, R₂ as the combinations (S+P) of those light components.These split light beams are incident on the photo detecting elements 224to 228 of the photo detecting unit 223, in the form of a parallelogram.

The photo detecting unit 223 is constructed as shown in FIG. 25. In thephoto detecting unit 223, a first photo detecting element 225 receivesthe reflecting light beam R₁ of three reflecting light beams R₁, R₁₁,R₂₁, which are split by the 3-beam Wollaston prism 218 and the +1storder (of diffraction) reflecting light beams. A second photo detectingelement 226 receives the reflecting light beam R₂ of those threereflecting light beams R₂, R₁₂, R₂₂, which are split by the 3-beamWollaston prism 218 and the −1st order reflecting light beams. A thirdphoto detecting element 224 consists of 4-divided photo detectingelements 224a to 224d which receive in divided form the centralreflecting beams R₀ of three reflecting light beams R₀, R₁₀, R₂₀, whichare split by the 3-beam Wollaston prism 218 and the 0-th orderreflecting light beams. Paired fourth photo detecting elements 227 and228 receive respectively the reflecting light beams R₁₀ and R₂₀, locatedon both sides of the central reflecting beam R₀. These photo detectingelements 224a to 224d, and 225 to 228 produce photo detecting signalsrepresentative of intensities of the detected reflecting light beams andtransfers them to a signal processing unit, not shown. Usually, thearray of the reflecting light beams R₁₀ and R₂₀ are perpendicular tothat of the reflecting light beams R₁ and R₂, as shown in FIG. 5. Inthis embodiment, however, the angle of these arrays is not a rightangle. The reason for this follows. The beams deformed through theastigmatism of the polarizing light beam splitter 216 are directed asindicated by dotted lines in FIG. 25. Because of this, the dividing lineof the photo detecting element 224 is oblique. Usually, the photodetecting elements 225 and 226 for receiving the light beams for forminga tracking error signal are located on the extension of the dividingline. It is for this reason that the light beams arrays are disposed atan angle, not right angle.

As shown in FIG. 26, the reflecting light beams R₀, R₁, R₂ going to thepolarizing light beam splitter 216 are convergent. Because of this, alight beam S₂ below than the optical axis is more greatly refracted bythe polarizing light beam splitter 216 than a light beam S₁ higher thanthe optical axis. Therefore, a focal point P on the Y-Z plane is at aposition displaced a distance y below the optical axis. The light beamsS₁ and S₂ are respectively in parallel with the incident light beamswhen these beams emanate from the polarizing light beam splitter 216.However, these beams, which should be focused at a point Q, areconverged at the point P displaced to the right from the point Q sincethese are displaced by the refraction. Since the influence of therefraction by the polarizing light beam splitter 216 on the change ofthe optical paths of the light beams on the X-Z plane is negligible, thepaths of the light beams are indicated by one-dot chain lines after thebeams are incident on the polarizing light beam splitter 216. It can beconsidered that the focusing point lies at the point Q on the X-Z plane.Planes perpendicular to the optical axis which contain the points P andQ are denoted as c and a, respectively. A plane as the perpendicular atsubstantially about midpoint between these planes c and a is denoted asb. The spots (images) in these planes a, b, and c are a circle, and ovalsegments being perpendicular to each other, respectively as shown inFIG. 27A, 27B, 27C.

Accordingly, if the objective lens 211 is apart from the magneto-opticaldisk D, the spot is a horizontal ellipse in the plane c as shown in FIG.27B. The value Fe (Ra+Rc)−(Rb+Rd) of the focus error signal has a minussign. If the objective lens 211 approaches to the magneto-optical diskD, the spot is a vertical ellipse in the plane a as shown in FIG. 27C.The value Fe of the focus error signal has a plug sign.

Accordingly, the result of calculating the equation (1) shows a state ofthe objective lens 211, focus, too near to the disk, or too far from thedisk, as in the fifth embodiment. As seen from FIG. 21, one can know adegree of defocusing of the objective lens 211 from the signal value Fe.

The operation of the sixth embodiment will be described.

The laser beams B emitted from the light source 214 are split into firstto third illuminating light beams B₀, B₁, B₂ by the diffraction grating215, and these split light beams are directed toward the objective lens211 by the polarizing light beam splitter 216.

The objective lens 211 converges the illuminating light beams B₀, B₁, B₂on the recording surface of the magneto-optical disk D.

Thus, the light beams B₀, B₁, B₂ are incident on the recording layer ofthe magneto-optical disk D, and reflected thereon. The reflecting lightbeams R₀, R₁, R₂, which are turned in the plane of polarizationdepending on the content of the data recorded, are incident again on theobjective lens 211.

The reflecting light beams R₀, R₁, R₂ emanate from the objective lens211, pass through the polarizing light beam splitter 216, and areincident on the 3-beam Wollaston prism 218.

The 3-beam Wollaston prism 218 splits each of those three light beamsinto three light beams, totally nine light beams R′ (R₀, R₁, R₂, R₁₀,R₁₁, R₁₂, R₂₀, R₂₁, R₂₂), and these split beams are incident on thephoto detecting unit 223.

The photo detecting elements 224a to 224d, and 225 to 228 of the photodetecting unit 223 produce photo detecting signals representative ofintensities of the detected reflecting light beams and transfers them tothe signal processing unit.

The signal processing unit generates a tracking error signal, a focusingerror signal, and a magneto-optical signal using the photo detectingsignals from the photo detecting unit 223, and transfers these signalsto the control unit.

The control unit receives a tracking error signal and a focusing errorsignal from the signal processing unit, and controls the lens drive unitin accordance with these signals, and hence moves the objective lens 211in the focusing direction Z and the tracking direction, thereby properlypositioning the objective lens 211. The magneto-optical signal is usedas a reproduced data signal.

The photo detecting unit 223 may also be constructed as shown in FIG.28. In the photo detecting unit 223, a first photo detecting element225′ receives the reflecting light beams R₁, R₁₁, R₂₁. A second photodetecting element 226′ receives the reflecting light beams R₂, R₁₂, R₂₂.A third photo detecting element 224 consists of 4-divided photodetecting elements 224a to 224d which receive in divided form thecentral reflecting beam R₀. Paired fourth photo detecting elements 227′and 228′ receive respectively the reflecting light beams R₁₀ and R₂₀.With this construction, the tracking error signal is more intensified.

It should be understood that the present invention is not limited to thespecific embodiments as mentioned above, but may variously be changed,modified, and altered within the scope of the invention. For example,the optical pickup device of the invention may be applied to amagneto-optical disk for mini-disk as a recording medium as well as amagneto-optical disk for MO. In the optical system, the beam splitterfor splitting the illuminating light beams and the Wollaston prism maybe formed in a single unit construction viz., the multi-functionalWollaston prism.

As seen from the foregoing description, the collimatorless opticalsystem is employed for the magneto-optical recording/reproducing pickupdevice of the invention. Accordingly, the optical pickup device may beconstructed not using the collimating lens and the converging lens. Theresult is reduction of the number of required parts, simplifiedconstruction, and realizing of a compact and low-cost optical pickupdevice for magneto-optical disk. In the optical pickup device using theWollaston prism, the angle adjustment is easier than in the opticalpickup device using the plate analyzer such as 18 shown in FIG. 2. It isavailable at low cost. Further, there is no need of using the half-waveplate in constructing the optical pickup device. This feature alsocontributes to the cost reduction.

Reduction of the distance between the light source and the polarizingbeam splitter is possible by employing the positive singlet. Because ofthis, the size reduction of the optical pickup device and an efficientuse of the light beam emitted from the light source as well arerealized. For the aberration correction, the positive singlet may beused in addition to the objective lens. This indicates an easyaberration correction in the overall optical system of the pickupdevice.

Since the multifunctional Wollaston prism having both functions of thepolarizing beam splitter and the cylindrical lens is used, the number ofrequired parts is reduced. Further, the positive singlet is providedbetween the light source and the Wollaston prism. Accordingly, theoptical length is reduced. This leads to the size and cost reduction ofthe optical pickup device, an efficient use of the light beam emittedfrom the light source, and an increase of the output power to the disk.For the aberration correction, the positive singlet may be used inaddition to the objective lens. This indicates an easy aberrationcorrection and easy design of the overall optical system of the pickupdevice. The converging position of the light beams on the photodetecting elements of the photo detecting unit may be adjusted by movingthe positive singlet along the optical axis. The structure is simplerand the position adjustment is easier than in adjusting component withan electrical wiring, such as the photo detecting unit.

Since the diffraction grating and the positive singlet may be formed ina single construction, the assembling work and the adjustment are easy,the construction is simple, moreover.

At least, three reflecting light beams are incident on each of the firstand second photo detecting elements. Accordingly, a photo detectingelement which can produce a large tracking error signal is realized.

At least, three reflecting light beams are incident on each of the firstand second photo detecting elements. Accordingly, an optical pickupdevice which can produce a large tracking error signal is realized.

A ratio of the 0-th order (of diffraction) light beam to the light beamsof the ±1st order light beams is selected within a preset range.Therefore, the present invention succeeds in providing an optical pickupdevice which can increase the amplitudes of the magneto-optical signaland the focusing error signal.

Since the basic split-light quantity ratio by the 3-beam Wollaston prismis selected within a preset range, an optical pickup device of thepresent invention can produce a magneto-optical signal having largeamplitudes.

An angle between a line connecting the centers of the first and secondphoto detecting elements and a line connecting the paired fourth photodetecting elements are not a right angle. Accordingly, a photo detectingelement well adaptable for an optical system in which the direction ofthe splitting by the diffraction grating is at an angle, not a rightangle, to the direction of the splitting by the Wollaston prism, isrealized. Additionally, use of the plate beam splitter reduces the sizeof the optical pickup device.

What is claimed is:
 1. A magneto-optical recording/reproducing pickupdevice without a collimator, comprising: a light source for generatinglight beams; a diffraction grating for splitting the light beam emittedfrom the light source into at least three light beams; an objective lensfor converging at leas three light beams emitted from the light sourceon a magneto-optical recording medium where the at least thee lightbeams are at least partially reflected forming reflecting light beams,and receiving the reflecting light beams from the recording medium, themagnification of the objective lens being −6.0 to less than −12.0 whenan object point lies at a signal surface of the magneto-opticalrecording medium; a beam splitter for separating the light beams comingfrom the light source and incident on the objective lens from the lightbeams coming through the objective lens; a Wollaston prism for splittingthe reflecting light beams coming from the magneto-optical recordingmedium through the beam splitter; and a photo detecting unit including aplural number of photo detecting elements for detecting the light beamsemanating from the Wollaston prism.
 2. The magneto-opticalrecording/reproducing pickup device as claimed in claim 1, furthercomprising a positive singlet for reducing a degree of divergence of thelight beams diverging from the light source, which is provided betweenthe light source and the objective lens, the total magnification of thedevice including the objective lens and the positive singlet being −3.0to −6.0.
 3. The magneto-optical recording/reproducing pickup device asclaimed in claim 2, wherein said diffraction grating and said positivesinglet are formed in a single construction which is positionallyadjustable in a turning direction and an optical axis direction.
 4. Themagneto-optical recording/reproducing pickup device as claimed in claim1, wherein a concave lens, movable along an optical axis for focusingthe reflecting light beams on the photo detecting element, is providedbetween the Wollaston prism and the photo detecting element.
 5. Themagneto-optical recording/reproducing pickup device as claimed in claim1, wherein the diffraction grating splits the light beams from the lightsource into the light beams of ±1st order of diffraction and the lightbeams of 0-th order of diffraction, each of these light beams beingsplit into at least three light beams by the Wollaston prism, the lightbeams of at least 3×3 being incident on the photo detecting element ofthe photo detecting unit in the form of a parallelogram, the photodetecting unit includes a first photo detecting element for receivingthe light beams of the +1st order of diffraction, a second photodetecting element for receiving the light beams of the −1st order ofdiffraction, a third photo detecting element, consisting of a pluralnumber of photo detecting elements, for receiving in divided form thecentral light beam of the three light beams on the 0th order ofdiffraction, and a pair of fourth photo detecting elements for receivingrespectively the reflecting light beams located on both sides of thecentral reflecting beam of the three light beams split by the Wollastonprism of the 0-th order, and an angle between a line connecting thecenters of the first and second photo detecting elements and a lineconnecting the paired fourth photo detecting elements is not a rightangle.
 6. The magneto-optical recording/reproducing pickup device asclaimed in claim 1, wherein the diffraction grating splits the lightbeams from the light source into the light beams of ±1st order ofdiffraction and the light beam of 0-th order of diffraction, each ofthese light beams being split into at least three light beams by theWollaston prism, the light beams of at least 3×3 being incident on thephoto detecting elements of the photo detecting unit, and the photodetecting unit includes a first photo detecting element for receiving atleast three light beams of the +1st order of diffraction, a second photodetecting element for receiving at least three light beams of the −1storder of diffraction, a third photo detecting element, consisting of aplural number of photo detecting elements, for receiving in divided formthe central light beam of the three light beams of the 0-th order ofdiffraction, and a pair of fourth photo detecting elements for receivingrespectively the reflecting light beams located on both sides of thecentral reflecting beam of the three light beams split by the Wollastonprism of the 0-th order of diffraction.
 7. The magneto-opticalrecording/reproducing pickup device as claimed in claim 6, wherein theratio of the quantities of the 0th order to ±1st order of diffraction,caused by the diffraction grating, is set within a range of 8.5 to 15.8. The magneto-optical recording/reproducing pickup device as claimed inclaim 6, wherein the Wollaston prism splits the received light beamsinto an ordinary ray, an extraordinary ray, and a light beam as thecombination of the ordinary ray and extraordinary ray, and the ratio ofthe quantities of the ordinary ray and the extraordinary ray to thewhole light quality is within 30 to 45%.
 9. The magneto-opticalrecording/reproducing pickup device according to claim 1, in which themagnification of the objective lens is within −7.0 to −9.0.
 10. Amagneto-optical recording/reproducing pickup device without a collimatercomprising: a light source for generating light beams; a diffractiongrating for splitting the light beam emitted from the light source intoat least three light beams; an objective lens for converging at leastthree light beams emitted from the light source on a magneto-opticalrecording medium where the at leas three light beams are at leastpartially reflected forming reflecting light beams, and receiving thereflecting light beams from the recording medium, the magnification ofthe objective lens being −6.0 to less than −12.0 when an object pointlies at a signal surface of the magneto-optical recording medium; aWollaston prism for causing an astigmatism including a polarizing lightsplitting film for separating the light beams coming from the lightsource and incident on the objective lens for the light beams comingthrough the objective lens, the Wollaston prism being composed of afirst prism joined to a second prism made of crystal, a plane includingthe optical axis of the reflecting light beam coming in through theobjective lens and the optic axis of the first prism, is at an angle,not a right angle, to a plane including that optical axis and the opticaxis of the second prism, the Wollaston prism is disposed in the opticalpath of the reflecting light beams as light beams being convergent andnot parallel in a state that is slanted with respect to the opticalaxis, whereby causing an astigmatism, a positive singlet providedbetween the light source and the objective lens, the total magnificationof the device including the objective lens and the positive singletbeing −3.0 to −6.0; and a photo detecting unit including a plural numberof photo detecting elements for detecting the light beams emanating fromthe Wollaston prism.
 11. The magneto-optical recording/reproducingpickup service device as claimed in claim 10, wherein the diffractiongrating and the positive singlet are formed in a single constructionwhich is positionally adjustable in a turning direction and an opticalaxis direction.
 12. The magneto-optical recording/reproducing pickupdevice as claimed in claim 10, in which the diffraction grating splitsthe light beams from the light source into light beams of ±1st order ofdiffraction and a light beam of 0-th order of diffraction, each of theselight beams being split into at least three light beams by the Wollastonprism, the light beams of at least 3×3 being incident on the photodetecting elements of the photo detecting unit in the form of aparallelogram, the photo detecting unit includes a first photo detectingelement for receiving the light beams of the +1st order of diffraction,a second photo detecting element for receiving the light beams of the−1st order of diffraction, a third photo detecting element, consistingof a plural number of photo detecting elements, for receiving in dividedform the central light beam of the three light beams of the 0-th orderof diffraction, and a pair of fourth photo detecting elements forreceiving respectively the reflecting light beams located on both sidesof the central reflecting beam of the three light beams split byWollaston prism of 0-th order of diffraction, and an angle between aline connecting the centers of the first and second photo detectingelements and a line connecting the paired fourth photo detectingelements is not a right angle.
 13. The magneto-opticalrecording/reproducing pickup device as claimed in claim 10, wherein thediffraction grating splits the light beams from the light source intolight beams of ±1st order of diffraction and a light beam of 0-th orderof diffraction, each of these light beams being split into at leastthree light beams by the Wollaston prism, the light beams of at least3×3 being incident on the photo detecting elements of the photodetecting unit, and the photo detecting unit includes a first photodetecting element for receiving at least three light beams of the +1storder of diffraction, a second photo detecting element for receiving atleast three light beams of the −1st order of diffraction, a third photodetecting element, consisting of a plural number of photo detectingelements, for receiving in divided form the central light beam of thethree light beams of the 0-th order of diffraction, and a pair of fourthphoto detecting elements for receiving respectively the reflecting lightbeams located on both sides of the central reflecting beam of the threelight beams split by the Wollaston prism of the 0-th order ofdiffraction.
 14. The magneto-optical recording/reproducing pickup deviceas claimed in claim 13, wherein the ratio of the quantities of the 0-thorder to ±1st order of diffraction, caused by the diffraction grating,as set within a range of 8.5 to
 15. 15. The magneto-opticalrecording/reproducing pickup device as claimed in claim 13, wherein theWollaston prism splits the received light beams into an ordinary ray, anextraordinary ray, and a light beam as the combination of the ordinaryray and extraordinary ray, and the ratio of the quantities of theordinary ray and the extraordinary ray to the whole light quantity iswithin 30 to 45%.
 16. The magneto-optical recording/reproducing pickupdevice as claimed in claim 10, wherein the magnification of theobjective lens is within −7.0 to −9.0.
 17. A photo detecting unit forreceiving the reflecting the light beams formed in a manner that a lightbeam is split into at least three light beams of 0-th order and ±1storder of diffraction by a diffraction grating, the split light beams areincident on a recording medium without using a collimator, the lightbeams reflected on the recording medium are each split into at leastthree light beams by a Wollaston prism, said photo detecting unitcomprising: a first photo detecting element for receiving the lightbeams of the +1st order of diffraction; and a second photo detectingelement for receiving the light beams of the −1st order of diffraction.18. A magneto-optical recording/reproducing pickup device without acollimator comprising: a light source for generating light beams; adiffraction grating for splitting a light beam emitted form the lightsource into at least three light beams of 0-th order and ±1st order ofdiffraction; an objective lens for converging the at least three lightbeams split by the diffraction grating on a recording medium, andreceiving the reflecting light beams from the recording medium where theat least three light beams are at least partially reflected formingreflecting light beams; a Wollaston prism for splitting the reflectinglight beams coming in through the objective lens into at least threelight beams; a photo detecting unit for receiving at least ninereflecting light beams from the Wollaston prism, said photo detectingunit including a first photo detecting element for receiving at leasthree light beams of the +1st order of diffraction, a second photodetecting element for receiving at least three light beams of the −1storder of diffraction, a third photo detecting element, consisting of aplural number of photo detecting elements, for receiving in divided formthe central light beam of the three light beams of the 0-th order ofdiffraction, and a pair of fourth photo detecting elements for receivingrespectively the reflecting light beams located on both sides of thecentral reflecting beam of the three light beams split by the Wollastonprism of the 0-th order of diffraction; and lens drive means for drivingthe objective lens for positioning adjustment.
 19. The magneto-opticalrecording/reproducing pickup device as claimed in claim 18, wherein theratio of the quantities of the 0th order to ±1st order of diffraction,caused by the diffraction grating, is set within a range of 8.5 to 15.20. The magneto-optical recording/reproducing pickup device as claimedin claim 18, wherein the Wollaston prism splits the received light beamsinto an ordinary ray, an extraordinary ray, and a light beam as thecombination of the ordinary ray and extraordinary ray, and the ratio ofthe quantities of the ordinary ray and the extraordinary ray to thewhole light quantity is within 30 to 45%.